Microwave directive antenna



Feb. 20, 1951 P. H. SMITH MICROWAVE DIRECTIVE ANTENNA 2 SheetS Sheet 1 Filed Aug. 14, 1943 INVENTIOR By P. H. SM/ TH Nut N M A T TORNE V ANTENNA 12 AT LEFT R RIGHT 3 AND 69 2 Sheets-Sheet 2 FIG. 6

D/RECWVE CHARACTER/377C ma/ve'r/c PLANE CON/CAL SCANNING MAX/MUM OIPE C 7' lVE LOBE H L T A m A N m m A ELECTRIC PLANE Feb. 20, 1951 Filed Aug. 14, '1943 ANTENNA I2 ATR/GHT.

m S a m R 3 mm 2M 3 NM .5 w n mm A M m n m INVENTOR By RH, SM/ m A 7' Tom/Ev Patented Feb. 20, 1951 MICROWAVEDIRECTIVE ANTENNA Phillip H. Smith, Denville, N. .L, assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., .a corporation of New York Application August 14, 1943, SerialNo. 498,622

16 Claims. (Cl. 250-33.65')

I This invention relates to directive antenna systems and particularly to microwave conical scanning radar or direction-finding antennas.

As disclosed in Patent 2,083,242 to W. Rlnge, British Patent 4.50.484 and the copending' application of C. B. H. Feldman, Serial No. 451,932, filed July 22, 1942, now Patent 2,419,556, issued April 29, 1947, it has been proposed to secure, in a, direction-finding system employing ultra short, decimetric or centimetricwaves, lobe rotation about a minor lobe .axis, that is, conical scanning, by utilizing a dipole and a rotating reflector. While these prior art conical scanning systems may be successfully used, they are not entirely satisfactory, and it, now appears desirable toobtain a conical scanning antenna hav-' ing distinct mechanical and electrical advantages not present in the prior art, arrangements,

It is one object of this invention to determine accurately the propagation direction of a radio wave.

It is another object of this invention to secure lobe rotation, without utilizing a rotating reflector.

It is another object of this invention to adjust in a conicalscanning system, the, angle between the, principal axis of the maximum antenna lobe and the antenna axis.

It. is another objectof this invention to obtain a high gain conical scanning antenna.

It is another object of, this invention to obtain, in a conical scanning system, a rotating directive characteristic having negligible minor lobes...

It is. another object of this invention to utilize, in a conical scanning antenna system, simple light weight apparatus for producing the lobe rotation.

It is another object of this invention to obtain, in a{ conical scanning system, truly circular scanning and a constant con-e angle in all planes containing the antenna axis.

It is another object of this invention to couple efficiently a rotating coaxial line inner conductor andia stationary coaxial line conductor.

It is another object of this invention to match impedances throughout a system comprising a rotating antenna, a rotating line conductor and a translation device.

It is still another object of this invention to adjust the length of av coaxial line without using sliding contacts;

In accordance, with one embodiment of the invention, a quarter wave primary antenna extends perpendicular to the axis of a paraboloidal antenna, the half-dipole being in the focal plane of the reflector and its geometrical midpoint being spaced from the'refiector focus. This primary ant nna is connected to the farend terminal of the inner conductor of a coaxial line which extends along the reflector axis and through the reflector apex',,and means are provided for rotating the inner conductor wher by the antenna rotates like awheel-spoke and the antenna mid-point describes 'a circle about the reflector focus. The near-end of the coaxial line is connected to a translation device such as a radar transceiver, the transceiver being coupled to the rotating inner conductor through a lowimpedance rotatable junction. In operation, the semi-dipole illuminates the entire reflector and the principal axis of the maximum lobe of the overall directive characteristic describes in space a circular cone, the axis of which is aligned with the reflector axis. Since the linear antenna rotates, the so-called electric and magnetic planes rotate, and hence, rotary (not circular) polarization is utilized. An auxiliary plane reflector is positioned in front of the quarter-waveprimary antenna so as to face the parabolic reflector, for the purpose of increasing the gain of the system. Also, an annular metallic reflector is positioned on the reflector axis between the quarter-wave antenna and the paraboloid reflector, whereby theintensities of the minor lobes are reduced.

The invention will be more fully understood from a perusal of the following specification taken in conjunction with the drawing on which like reference characters denote elements of similar function and on which:

Fig. 1 is a diagrammatic longitudinal crosssectional View:

Fig. 2 is a diagrammatic transverse crosssectional view, of the preferred embodiment of the invention;

Fig. 3 is a detail view of the primary antenna included in the preferred embodiment;

Fig. 4' is a theoretical curve;

Figs. 5, 6 and 7 are measured curves used in explaining the invention; and

Fig. 8 is a cross-sectional view of a front concave reflector which may be used in place of the front plane reflector included in the system ofFigs. 1 and 2. I

Referring to Figs. 1, 2 and 3, reference numeral I denotes a paraboloidal reflector or secondary antenna, having an apex 2, a principal axis 3, a focus 4 and a focal plane 5. Numeral 6 denotes a shaft extending through apex 2 and aligned with axis 3, the shaft being driven by motor 1 and supported near each end by a bearing 8. Reference numerals 9 denote retaining members for maintaining the bearings 8 in their proper positions. The rotating shaft 6 constitutes the inner conductor of a coaxial line It! having an outer conductor H which does not rotate. Numeral l2 denotes a quarter-wave primary antenna member extending in focal plane in a direction perpendicular to axis 3 and equipped at its far end with an anti-corona knob l3. The antenna I2 is connected to shaft 6 through a solid conical metallic member I4 which is integral with shaft 6 and has a threaded socket l5 for receiving the threaded base l6 of antenna 2 as shown in Figure 3. Hence, the entire conductor 29 constitute an open-ended quarterwave line having an input impedance Z1. The outer surface of sleeve 32, the annular end-piece 34 and the inner surface of sleeve 33 form a short-circuited quarter-wave line having an input impedance Z2 equal to infinity; and the outer surface of sleeve 33 and the corresponding inner quarter-wave antenna l2 and, in particular, its

base l3 and its geometrical mid-point H, are adjustably spaced from the reflector focus 4. Numeral I8 denotes a plane reflector extending perpendicularly to axis 3 and supported through a high impedance or short-circuited quarterwave line W by shaft 3. The high impedance IS in effect insulates reflector 3 from shaft 6. The line l9 comprises a quarter-wave sleeve member 2 0, the adjacent tubular surface of inner conductor 5 and an annular end-member 2| which conductively connects one extremity of sleeve to shaft 6. As shown in Fig. 2, reflector l8 has two curved edges and its center point is offset from its center of rotation.

The coaxial line It is connected to a translation device 22 through a main coaxial line 23 comprising an inner conductor 24 and an outer conductor 25, the outer conductors I! and 25 of lines I!) and 23 being directly connected and the inner conductors 24 and 5 of these lines being coupled through a quarter-wave coupling line 26 comprising the quarter-wave sleeve 21 and the corresponding tubular portion of shaft 6. Device 22 may be a transmitter or a receiver or a transceiver. As viewed from either end of line 23, the capacity between the surface of the inner conductor 3 and the inner surface of sleeve 21 is exceedingly large and approaches infinity, since line 26 is a quarter-wavelength long, and the impedance between these surfaces approaches zero. Hence the rotating shaft 6 and the stationary coupling sleeve 2'! spaced therefrom are connected through a low impedance at the operating frequency. Reference numeral 28 denotes a stub tuning coaxial line which is connected to line H] at a point opposite line 23 and which comprises an inner conductor 29 and an outer conductor 33. The conductors 29 and 3[! are connected respectively to outer line conductor H and sleeve 2?. Numeral 3! denotes a plunger which is for all practical purposes air insulated from the line conductors 29 and 3c and which comprise a pair of quarter-wave coaxial sleeves 32 and 33 coaxially included between and spaced from the stub line conductors 29 and 33 and an annular end-piece 34 connecting the sleeves together at one end and also spaced from conductors 29 and 30. Numeral 35 denotes a handle attached to plunger 3| and extending through a longitudinal slot in the outer conductor for permitting longitudinal adjustment of the plunger, and numeral denotes a saddle or supporting member which ridesin the slot and slidably secures handle 35 and plunger 3| to the outer coaxial line conductor 30.

As viewed from the junction 26 or near end of the stub line 28 the inner surface of sleeve 32 and the corresponding surface of inner line surface of the outer line conductor 30 constitute an open quarter-wave line having an impedance Z3. Hence, at the input end of the plunger 3!, the stub line coaxial conductors 29 and 38 are connected together through'three serially connected impedances Z1, Z2 and Z3 and, since one of these impedances, Z2, is infinity, the stub line is effectively open-circuited and appears to be ended or cut off at this point, regardless of the value of Z1 and Z3. By moving the handle 35 the length of the stub line, and therefore its impedance value, may be adjusted. Since the sleeves 32 and 33 and the end piece 34 are, except at the saddle member 38, air-insulated from conductors 29 and 30, sparking during movement of plunger 3i is completely eliminated, the current through saddle 36 being negligible. It may be noted here that, if desired, the plunger 3! may be reversed and utilized to short-circuit a coaxial line. Thus, as viewed from the far end of stub line 28, the impedance connecting the stub line conductors 29 and 30 comprises two open-ended quarter-wave lines having input impedances Z4 and Z5, the inputs of these quarterwave lines being directly connected in series through the annular member 34 which has an impedance Z6 equal to zero. Since each of these open-ended quarter-wave lines is terminated in an impedance Z3 or Z1, which is in series with an impedance Z2 having a value of infinity, the quarter-wave lines are in effect each terminated in an infinite impedance, so that the input impedances Z4 and Z5 are each equal to zero. Hence the conductors 29 and 30 are, at the point adjacent to end-piece 34 and as viewed from the far end of the stub line, connected together or short-circuited through three serially connected impedances each equal to zero.

The primary antenna I2 is coupled to line l0 through a pair of serially connected quarterwave impedance transformers 31 and 38. The transformer 3! comprises the inner surface of the flared quarter-wave tubular conductor 39 and enclosed conical surface 40 of member l4; and the transformer 38 comprises the inner surface of the quarter-wave tubular conductor 4| and the corresponding portion of the shaft or inner line conductor 6. Numeral 42 denotes a slidable quarter-wave sleeve havin a transverse slot 44 for permitting adjustment by a screwdriver, and numeral 43 denotes a stationary quarter-wave sleeve, the sleeves 42 and 43 being spaced about a quarter-wavelength. Each of sleeves 42 and 43 is equipped with annular endpieces 45 which connect the sleeve and conductor 6. Numeral 46 denotes a longitudinal slot in the outer conductor H and numeral 41 designates a tubular slidable member for covering slot 46. As is believed to be apparent, the outer surface of sleeve 42 and the corresponding inner surface of the outer line conductor ll constitute one quarter-wave impedance transformer 48, and the outer surface of sleeve 43 and the corresponding inner surface of the outer conductor H constitute another quarter-wave impedance transformer 49. Also, the outer surface of the quarter-wave coupling of sleeve 21. and the associated innerlsurfaceof line conducamass;

tor= I-I -constitute a quarter-wave impedance transformer 50, the spacing between transformers '49 and 50. being negligible. x K

The impedance transformers function to match the impedances throughout the transmission system. Thus, the impedance Zr of the primary antenna I2 is changed by the flared impedance transformer 31 having a characteristic impedance Z'a, into animpedance Z9 at the input of transformer 31. The impedance Z9 is changed by the transformer 38 having a .characteristic impedance Z19 into an impedance Zn at the input of transformer 38, the impedance Z11 being equal to the characteristic impedance Z12 of the coaxial line I0. The line impedance Z12 is transformed by a quarter-wave line 48 having ia-characteristic impedance Z13 into an impedance Z14 at its input end. Since the spacing between sleeves 42- and 43 is a quarter-wavelength, the impedance Z14 is changedby the line characteristic impedance Z19 into an impedance Z15 at the .output end of the transformer 49. The impedance Z15 is changed by the transformer 49- having a characteristic impedance Z16 into an impedance Zn at the input end of transformer 49:1and at the output end of transformer 50, the spacing between transformers 49 and 50 being negligible. The impedance Zi'z is changed by transformer 50 which has a characteristic impedance Z18 into an impedance Z19, the impedance Z19 being equal to the characteristic impedance Z20; of the main-coaxial line, 23. In one. actual embodiment constructed and'tested, the valuesrof Z: to Z20, inclusive, were respectively 36., 28', 23:, 36, 57, 57, 35.5, 22, 145',v 24, 5.5, 20, 75 and 75 ohms. The stub line 28- is in practice adjusted to tune out any residual mismatch or reactance. at the point where lines II] and 23 are connected through the coupling sleeve 21.

As illustratedin Fig. 1, line If! comprising shaft 6 and the outer conductor II is. equipped with several high impedances of the short-circuited quarter-wave line type; such as impedance I9.: Thus, reference numeral 51: denotes a highimpedance comprising the quarter-wave sleeve '52, the adjacent quarter-wave portion of shaft'fi' and end-piece 53 connecting sleeve 52 and shaft 6. The impedance I functions to insulate the portion of shaft 6 associated with reflector I8 from the primary antenna I2. Numeral 5'4 denotes a high impedance'comprising the quarter-wave sleeve 55, annular member 56 and the outer surface of the quarter-wavetubular conductor 4|. This impedance resembles the well-known balance-to-unbalance coupling and functions to prevent antenna I2: fromestablishing standing waves onthe outer, surface of line conductor II.. Numeral 5'! denotes another shortcircuited quarter-wave line. comprising sleeve 58.,

. annular member 59 and the corresponding portion of shaft 6. Transformer 51 sectionalizes shaft 6 and presents a high impedancev to the waves conveyed. alon the 90-degree bend connecting coaxial lines I0 and 23.

The rotating shaft 6, primary antenna I2 and plane reflector I8 are enclosed in a Plexiglas housin 63' which is attached to they paraboloidal reflector I and functions as a support for the system of Figs. 1",-2"and 3will'noW beexplained. Assuming motor 'I is operating at a constant speed, the mid-points of thequarter wave primary antenna-l 2 andthe associated plane reflector "I 8 are rotated about the focal axis 3. Considering 9 the transmitting operation, pulsed centimetric waves are supplied over lines .123 and It to the-primaryantenna I 2 and-waves polarized in the plane of the rotating half dipole -I-2 are radiated. The plane of polarization, which contains the linear antenna I2, and the plane perpendicular thereto, both rotate. These planes will behereinafter termed the electricand mag netic planes, respectively. The primary antenna functions to illuminate or energiz the paraboloidal reflector I. More particularly, certain of the waves-emitted b the primary antenna I2 pass directly to the secondary antenna I and,

bearing 8 at the far end of shaft 6. Numeral 6|v denotes an. annular or ring. reflector coaxially related to shaft 6 and positioned. between the primary antenna I2 and the paraboloidal reflector I.

Referring to Fig. 4 and assumingdevice 22 is a transceiver, theoperation of theradar' antenna in-a manner well understood in the art, these particular waves are redirected in substantially parallel directions making an angle a with the reflector axis 3. Certain other waves emitted by the primary antenna I2" impinge upon the plane reflector- I-8 andare reflected or directed toward the para-boloidal reflector, these waves being thence propagated or redirected by the paraboloidal reflector in the same common direction as the waves supplied directly to the paraboloi'dal reflector. The reflector IB- and primary antenna I2 are preferably spaced so as to obtain substantial phase agreementbetween the setof waves supplied directly and the set of waves supplied indirectly via reflector I6 tothe parabolic reflector; Preferably, the spacing should be such that the average difference in thepath'lengths of these two sets of waves is about a half-wavelength. The plane reflector functions to increase the gain of the system about 2 decibels.

In addition, a certain amount of energy radiated by the primary" antenna I2 is absorbedand reradiated, or simply reflected, by the ring reflector 6*], the phase and amplitude of this reradiated' energy being related to the size of'reflector BI and is position along the parabolic reflector axis 3. Ordinarily, the position of reflector BI is adjusted so that the phase and propagation direction of the waves radiated thereby are such as to cancel, at least in part, theminor'lob'esincluded in the directive characteristic of the antenna system formed by the primary antenna I2 and secondaryantenna I.

The primary antenna I2 andthe direction of maximum action for the entire antennasystem comprising dipole I2 and reflectors I and I8; are ordinarily on opposite sides'of the reflector aXis'3. Thus, with the primary antenna iz'inthe position. illustrated in Fig; 1', maximum action occurs in. the direction 62:. ,As theprimary antenna I2 and the associated plane reflector I8 :rotate, the direction 62' describes in space-a cone, the angle a of. which may be varied to some extent by'adj'usting the spacing between the primary antenna mid-point I1 and the reflector focus 4;" As shown. in Fig. 3, the above-mentioned spacing may be changed by adjusting the threaded connection I5, I6 at the base of antenna t2. In Fig. 4 the reference numeral 63 denotes the rotating maximum lobe having a principal axis 64, which is identical to. the direction 62* of the maximum action; and numeral 65 designates a circle representing the trace described by the rotating lobe axis 64 on a plane perpendicular to the reflector axis 3. Considered differently, the lobe 63 rotates about the so-called equi-intensity. line 66'. Numeral 6'! denotes the diametrically opposite posi tion for the maximum lobe and. numeral 68 denotes the so-called lobe cross-over point.

In reception, the echo pulsed waves reflected by the target are directively collected by the'antenna system and supplied over lines I and 23 to the receiver in the transceiver 22, the receiving action being the converse or reciprocal of the transmitting action. Assuming the target is on the cone axis 69, Fig. 4, which coincides with the reflector axis 3, the echo pulses have a constant intensity correspondin to the cross-over point 68. If the target is off the axis 69, the amplitude of the received echo pulsed waves varies sinusoidally, the frequency of the detected waves being dependent on the conical scanning rate and, therefore, on the speed of motor 1. With the target ofi the axis the amplitude of the echo waves is a measure of the degree of the pointing error,

that is, the angular deviation of the target from the reflector axis. The phase of the detected current, as compared to the phase of a current from a reference generator, is indicative of the radial direction, relative to the reflector axis, of the target. In one actual embodiment, a reference generator driven by motor 1 in synchronism with the antenna beam and a cathode tube indicator are utilized for ascertaining the amplitude and phase of the echo waves and, therefore, for ascertaining the pointing error of the antenna. The direction and amount of displacement of a dot pattern from the center of the cathode-ray tube screen represent the pointing error.

Figs. 5 and 6 illustrate, respectively, the measured electric plane and magnetic plane directive curves for a system constructed in accordance with the invention and illustrated by Figs. 1, 2 and 3. Thus, referring to Fig. 5, reference numeral I0 denotes the characteristic obtained with the primary antenna I2 at the right of the reflector axis 3 and numeral H designates the characteristic obtained with the linear antenna l2 at the left of the axis 3. It will be noted that the maximum lobe 12 of curve is at the left and the maximum lobe 13 of the curve H is at the right of the cone axis 69. Also it will be noted that, by reason of the use of the ring reflector 6|, the minor lobes T4 of curve 10 and the minor lobes 15 of curve H are negligible, the minor lobes for each curve being for the one-way trip approximately decibels down from the maximum value of the maximum lobe. For the round trip, that is, from the transceiver to the target and return, the minor lobes are .down40 decibels. Referring to Fig. 6, numeral 16 denotes the magnetic plane characteristic having a maximum lobe Ti and the negligible minor lobes 18, the maximum lobe 11 being aligned with the reflector axis 3 and the cone axis69. Stated differently, in a plane perpendicular to the linear antenna l2, the directivity is the same with the spoke antenna l2 pointing first in a given radial direction and then in the opposite direction. I 1

Fig. 7 illustrates a set of error curves for the system constructed and tested as described above, the curve each being representative of different amounts (but not directions) of pointing error. The error curves were obtained by rotating the antenna maximum lobe while maintaining the target fixed at a point on the circle 55 in Fig. 4. As illustrated by the curves of Fig. '7, with the target off the axis, the received intensity varies as a function of the beam rotation, that is, the envelope of the received current is related to the rotating speed of the quarter-wave antenna (2 Also the amplitude of the varying received current is a function of the error angle. Thus, with an angular pointing error of 2 degrees, a received current represented by curve is obtained and with a pointing error of 8 degrees a current represented by curve 8| is obtained, the amplitudes of each current being proportional to the amount of the pointing error. With the target aligned with the reflector axis 3, that is, with the pointing error equal to Zero degrees, a series of echo waves of constant envelope intensity is received as shown by the flat curve or envelope 19.

If desired, the plane reflector l8 of Fig. 1 may be replaced by a diflerent type of reflector, such as a concave reflector or a dipole reflector. In one embodiment actually constructed and tested, and comprising a relatively large paraboloidal reflector, a front reflector of the spherical type was utilized and the flared or conical member M was omitted, the antenna l2 being directly attached to the rotating shaft 6. Figs. 1 and 8 illustrate this embodiment, the structure shown in Fig. '8 being substituted for that shown at the right of the line XX in Fig. 1. In Fig. 8, reference numeral 82 denotes the spherical type reflector. This reflector rotates with shaft 6 but may be made tationary, if desired. The reflector 82 is isolated from shaft 5 by the high impedance circuit comprising quarter-wave line 19, while the direction 62 of action for the entire system is ordinarily on the opposite side of the reflector axis from the primary antenna l2. the action of the spherical reflector is sometimes such that the direction of maximum action and the primary antenna may be on the same side of the reflector axis 3.

Although the invention has been explained in connection with certain specific embodiments, it is to be understood that it is not to be limited to the embodiments described inasmuch as other apparatus may be successfully utilized in practicing the invention.

What is claimed is:

1. In combination, a paraboloidal reflector, an antenna element positioned in the focal plane of said reflector and. having one end or terminal at the reflector focus, substantially, and means for rotating said element about said terminal and reflector focus.

2. In combination, a paraboloidal reflector, a linear primary antenna positioned in the focal plane and having one terminal at the focus, and means for rotating said primary antenna about said terminal and focus.

3. In combination, a parabolic reflector, a primary antenna element positioned in the focal plane of the reflector, a ring reflector included between aid parabolic reflector and said primary antenna and symmetrically related to the axis of the parabolic reflector.

4. In combination, a parabolic reflector having a focus and a principal axis, an auxiliary reflector facing said parabolic reflector, a linear primary antenna positioned between said reflectors and spaced from said focus,said auxiliary reflector and primary antenna extending perpendicularly to said axis and means for rotating said primary antenna and auxiliary reflector about said axis.

.5. In combination, a parabolic reflector, a coaxial line comprising an inner and an outer conductor extending through said reflector, the inner conductor being aligned with the reflector axis, a linear quarter-wave primary antenna positioned in the reflector aperture, and connected to said inner conductor, means for rotating said inner conductor and a translation device connected to said line.

6. A combination in accordance with claim 5, the mid-point of said primary antenna being adjustably spaced from said inner conductor and the reflector focus.

7. A combination in accordance with claim ,5, a quarter-wave high impedance included between said primary antenna and said outer conductor.

8. A combination in accordance with claim 5, an auxiliary reflector positioned in front of said primary antenna, a high impedance, said auxiliary reflector being connected through said impedance to said inner conductor 9. A combination in accordance with claim 5, an auxiliary reflector positioned in front of said primary antenna and connected to said inner conductor and a high impedance comprising a portion of said inner conductor included between said primary antenna and said auxiliary reflector.

10. In an antenna system comprising a reflector having the shape of a surface of revolution about its directive axis, a directive antenna member located on said axis at substantially the focal said antenna member, means for causing radiation from said antenna at an angle with respect to said axis, and means for rotating said antenna member about said axis with respect to said reflector.

11. In combination, "a parabolic reflector having a focus and a principal axis, an auxiliary repoint of said reflector, and means for exciting flector facing said parabolic reflector, a linear" primary antenna positioned between said reflectors and spaced from said focus, said auxiliary reflector and primary antenna extending perpendicularly to said axis and means for rotating said primary antenna about said axis.

12. In an antenna system comprising a reflector having the shape of a surface of revolution about its directive axis and having a focal point on said axis, a concentric transmission line extending through said reflector and along said axis, the central conductor thereof extending beyond said focal point of said reflector, a radiating element attached at one end thereof to said central conductor near said focal point and extending therefrom transversely to said axis unsymmetrically with respect thereto and arranged for excitation from said transmission line, and a second reflector attached to said center conductor at a point beyond said focal point to reflect waves from said element toward said first reflector, and means to rotate said central conductor, radiating element, and second reflector as a unit to rotate the beam produced by said system about said axis.

13. In an antenna system comprising a reflector having the shape of a surface of revolution about its directive axis and having a focal point on said axis, a transmission line extending through said reflector and along said axis to said focal point, .a radiator-located near said focal point eccentrically with respect to said axis and 10 I connected for excitation from said transmissio line, and means to rotate said radiator about said axis.

14. In an antenna system comprising a reflector having the shape of a surface of revolution about its directive axi and having a focal point on said axis, a coaxial transmission line extending through said reflector and along said axis to the focal point thereof, a radiating element mechanically and conductively connected atone end thereof to the central conductor of said line and extending transversely thereto, and means to rotate said central conductor thereby to rotate said radiating element about said axis to rotate the field pattern produced by said radiating element and reflector about said axis.

15." In an antenna system comprising a reflector having the shape of a surface of revolution about its directive axis and having a focal point on said axis, a concentric transmission line extending through said reflector and along said axis, a radiating element attached to a central conductor of said transmission line near said focal point and extending therefrom transversely to said axis unsymmetrically with respect thereto and arranged for excitation from said transmission line, a econd reflector at a point beyond said focal point to reflect waves from said element toward said first reflector and means to rotate said central conductor and radiating element as a unit to rotate the beam produced by said system about said axis.

16. In an antenna system comprising a reflector having the shape of a surface of revolution about its directive axis and having a focal point on said axis, a coaxial transmission line extending through said reflector and along said axis to the focal point thereof, a radiating element mechanically and conductively connected to the central conductor of said line and extending therefrom unsymmetrically with respect to said axis and means to rotate said central conductor thereby to rotate said radiating element about said axis to rotate the field pattern produced by said radiating element and reflector about said axis.

PHILLIP H. SMITH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,828,705 Kolster Oct. 20, 1931 2,082,347 Leib et a1 June 1, 1937 2,083,242 Runge June 8, 1937 2,112,282 Fritz Mar. 29, 1938 2,118,419 Scharlau May 24, 1938 2,168,860 Berndt .1. Aug. 8, 1939 2,226,479 Pupp Dec. 24, 1940 2,237,792 Roosenstein Apr. 8, 1941 2,249,963 Lindenblad July 22, 1941 2,342,721 Boerner Feb. 29, 1944 2,422,361 Miller June 17, 1947 

